Process for data transmission by means of a geo-stationary satellite and at least one sub-satellite

ABSTRACT

A process for data transmission uses a geo-stationary satellite and at least one sub-satellite, the sub-satellites being placed on lower orbits. For the transmission of news, at least two data streams are transmitted with different powers. The first data stream, with a higher power is used for linking with the geo-stationary satellite (S 1 ) and the other data streams are transmitted by means of PN (pseudo-noise) sequences for linking with the sub-satellites (S 2 ). Two data transmissions thus take place in the same carrier frequency, but involving different satellites that are both within the visibility range of an earth station. This type of transmission using additional data streams of information is suitable for controlling and/or regulating the exchange of news between satellite-satellite and/or earth-satellite-earth or for controlling or regulating the orbit of the satellites.

FIELD OF THE INVENTION

The present relates to a process or method for data transmission bymeans of a geo-stationary satellite and at least one sub-satellite,whereby the sub-satellites are located on lower orbit paths.

DESCRIPTION OF THE PRIOR ART

News satellites generally travel along a geo-stationary circular orbitbath at an altitude of approximately 36000 km above the equator. There,the satellites have a strictly allotted orbital position so that thesame transmission frequency may be used for each satellite because ofthe distance between two neighboring satellites. Because thegeo-stationary orbit is slowly becoming filled, other satellite orbitpaths are being planned, for example, sub-satellite orbit paths with analtitude of a few hundred km or so-called quasi-stationary orbit paths.Details in this context are given in the literature, for example, in:

"P. Dondl: "LOOPUS erschliesst dem Satellitenfunk eine neue Dimension";NTZ-Archiv Vol. 5 (1983), Booklet 12, pages 327-335".

The gravest disadvantage therein is that different frequency ranges mustbe used to preclude mutual interferences. Namely, it is not simplypossible with conventional transmission methods to use the samefrequency for the geo-stationary satellite and for the sub-satellite,because this would disturb the geo-stationary satellite, particularlybecause during their visibility, satellites having lower orbital pathspass by several geo-stationary satellites (with respect to the radiatingdirection of the transmitting earth radio station) which in this caseuse the same frequency band. However, frequency ranges or transmissionband widths are strictly internationally regulated and distributed sothat they may not simply be used as desired. Thus, in order to providefor future communication requirements, additional possibilities must beprovided.

OBJECT OF THE INVENTION

It is the object of the invention to provide a method of the initiallydescribed type which makes it possible to make do with the current (orprescribed) frequency ranges.

SUMMARY OF THE INVENTION

According to the invention, this is achieved in that for communicationtransmissions at least two data streams of the same carrier frequency,but with different powers are used, whereby the first data stream with ahigher powers is used for linking with the geo-stationary satellite andthe further data streams are transmitted by means of PN (pseudo-noise)sequences and serve for linking with the sub-satellite orsub-satellites.

Thus, two data transmissions take place with the same carrier frequencywhereby, however, different satellites are served.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of the invention are given in the dependent claims and thedescription, in which an example embodiment is explained in detail inconjunction with the drawings, wherein:

FIG. 1 shows schematically the arrangement of the satellite orbitalpaths and the area of interference during transmission at the samecarrier frequency f_(B) ;

FIG. 2 shows schematically the principle of spreading by means of PNsequences;

FIG. 3a shows the conditions in the transponder of the geo-stationarysatellite without reception of the interfering power; and

FIG. 3b shows the conditions in the transponder of the geo-stationarysatellite with reception of the interfering or noise power.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BESTMODE OF THE INVENTION

FIG. 1 schematically shows the arrangement of the satellite orbit paths.The news satellite S₁ is located on a geo-stationary orbit path at adistance of approximately 36000 km altitude, and the sub-satellite S₂ islocated on a sub-satellite orbit path at a distance of approximately 400km altitude. Satellite S₂ receives data from an earth station E₁.Satellite S₂ receives data from a further earth station E₂. As can beseen in FIG. 1, an interference region is formed in which the two conesof the transmitting antennas overlap. For this reason, as initiallymentioned, it is not simply possible to use the same carrier frequencyfor both earth stations.

According to the invention this is, however, made possible in that twodata streams of the same carrier frequency, but of different powers areused, whereby the first data stream with a higher powers is used forlinking with a the geo-stationary satellite (S₁) and the further datastreams are transmitted by means of PN (pseudo-noise) sequences and areused for linking with the sub-satellite (S₂). By means of the powerdependent decoupling of the two satellite systems the same carrierfrequency range may be used without causing any interferences worthmentioning.

The structure of the PN sequences is composed according to FIG. 2 insuch a manner that they possess good correlation characteristics, whichmakes possible an exact detection of the signal. Thus, each data bit isexpanded or spread to the length of one PN sequence. If the PN sequencehas a length of, for example 1000 chips, then one data bit isrepresented by these 1000 chips. Thus, the power to be applied in thismanner only amounts to 1/1000 of the power which is required inconventional methods, whereby the two data streams are decoupled fromeach other by a factor of 1000 with respect to their powers.

If the same band widths are used, then the transmittable usable bit rateis reduced by the spreading factor relative to the conventional datastream due to the spreading of the transmission spectrum, that is tosay, due to the use of PN sequences.

In the following, it is assumed that a news link between ground stationsand a geo-stationary satellite S₁ (e.g. ECS) exists in the 14 GHzcarrier frequency range.

Now additionally, a news link at the same transmitting carrier frequencyof 14 GHz is to be established via a satellite S₂ which is located on adifferent orbital path, for example, on a circular orbital path of loweraltitude or on an elliptical orbital path.

Because of the lower altitude of the sub-satellite S₂, its range ofvisibility from a ground station E₂ is limited, that is to say, thesub-satellite S₂ is visible from the ground station E₂ for "n" minutesper revolution. The ground station E₂ must, with its antenna, fix on thesub-satellite S₂ at the rising horizon and must follow it up to thesetting horizon. Thereby, under the circumstances, the antenna scansover a certain region of the geo-stationary orbital path in which theother satellite S₁ is located and thereby interferes with its newstransmissions.

Let the ground station E₁ now transmit according to FIG. 3a, with a bandwidth of 10 MHz to the geo-stationary satellite S₁ and let the groundstation E₂ transmit with the same band width to the sub-satellite S₂.However, for transmitting, it uses a PN sequence with a spread factor of1000.

Because the spread factor is defined as the ratio of the length of thePN sequence to the duration of a data bit, the ground station E₂ canonly transmit a usable bit rate of approximately 10 KBits relative tothe 10 MBits of the ground station E₁. The necessary transmission powerfor the 10 KBits is, however, distributed over 10 MHz, whereby thesignal power is reduced so that within the 10 MHz band width of thegeo-stationary satellite S₁, its own signal is only interfered with bythe factor 1/1000. Thus, according to FIG. 3b only the base noise of thetransponder is increased.

Thus, two data transmissions take place in the same frequency range,whereby, however, different satellites are served, which satellites areboth located in the range of visibility of the earth station E₂.

The power relationships shown in FIG. 3 would only be valid if bothsatellites would have the same altitude. However, because thesub-satellite S₂ uses a lower altitude, a correspondingly lower powermust be radiated. That further reduces the interference power receivedby the geo-stationary satellite as shown in FIG. 3b, particularlybecause this interference power is generally damped by a factor ofapproximately 10⁺²⁰ due to the radiation reduction.

Especially advantageous applications of the invention are seen in thatinformation for controlling and/or regulating the communications flowbetween satellite-satellite and/or earth-satellite-earth or forcontrolling and or regulating the satellites with respect to theirorbital path, is transmitted by means of the additional data streams.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated, that it is intended tocover all modifications and equivalents within the scope of the appendedclaims.

I claim:
 1. A method for transmitting data by means of a geo-stationaryfirst satellite (S₁) on a geo-stationary orbit and at least one secondsatellite (S₂) on a different orbit at a smaller altitude than that ofsaid geo-stationary orbit, comprising the following steps:(a)transmitting, for communication purposes between transmitter groundstation means on earth and said first and second satellites, at leasttwo signal data streams having the same carrier frequency and differentpowers, (b) using a first signal data stream having a higher power forcommunication between said transmitter ground station means and saidgeo-stationary satellite (S₁), and (c) transmitting further signal datastreams having a lower power by means of PN sequences which spread saidfurther signal data streams within a length of said PN sequences forachieving said lower power to provide a power decoupling ofcommunications between said transmitter ground station means and saidgeo-stationary first satellite when communicating between saidtransmitter ground station means and said second satellite or secondsatellites, whereby said power decoupling is effective even if saidfirst and second satellites are in such positions relative to each otherand to said transmitter ground station means that an interference regionis formed by antenna cones of said ground station means.
 2. The methodof claim 1, wherein said further signal data streams having the lowerpower are used for transmitting control signals to said first and secondsatellites for keeping said first and second satellites on theirrespective different orbits.